Compact liquid crystal beam steering devices including multiple polarization gratings

ABSTRACT

Systems, methods, and apparatus are disclosed for attenuating an incident polarized light beam using a plurality of LCPGs and one or more switchable liquid crystal layers. When four LCPGs are used, a spacing between first and second LCPGs can be equal to a spacing between third and fourth LCPGs. Pi and FCL cells can also be used in place of more traditional LC switches. Switching of the LC switch can be imparted via an AC bias.

FIELD OF THE DISCLOSURE

The present disclosure relates to liquid crystal beam steering devicesand, more particularly, to switchable liquid crystal-based beam steeringdevices including multiple liquid crystal polarization gratings andrelated methods.

BACKGROUND OF THE DISCLOSURE

Recent advances in liquid crystal polarization grating (“LCPG”)technology have enabled the use of passive LCPGs, singly and incombination, to manipulate light, particularly in display applications(See, for example, U.S. Pat. No. 8,537,310 to Escuti, et al., which isincorporated herein in its entirety by reference). In general, passiveLCPGs possess a permanent, continuously varying periodic polarizationpattern to diffract incident light according to its polarization.

More recently, LCPGs have been combined with switchable liquid crystal(“LC”) devices to provide low Size, Weight, and Power (“SWaP”) beamsteering devices (See, for example, U.S. Pat. No. 8,982,313 to Escuti,et al., and Boulder Nonlinear Systems white paper, “Core Technologies,”September 2014,http://bnonlinear.com/wp-content/uploads/2014/09/Core-Technologies-White-Paper.pdf,accessed 30 Sep. 2015, which are incorporated herein in their entiretyby reference). As an example, by incorporating fast electro-optichalf-wave polarization retarders as a switch to control the handednessof polarization of the incident light, switchable beam steering deviceswith faster speed and lower SWaP compared to existing mechanicalsolutions, such as rotating Risley prisms, can be achieved.

As described, for example, in U.S. Pat. No. 8,537,310, U.S. Pat. No.8,982,313 and “Core Technologies” whitepaper, passive LCPGs generallyconsist of a nematic LC film that is surface aligned and UV-cured topresent a permanent, continuously varying periodic polarization pattern.The structure of such LCPGs provides an in-plane, uniaxial birefringencen that varies with position (i.e., n(x)=[sin(πx/Λ), cos(πx/Λ), 0], whereΛ is the period of the grating). Such transmissive gratings efficiently(e.g., with greater than 99% efficiency) diffract circularly polarizedlight to either the first positive or negative order, based on thepolarization handedness of the incident light.

As used herein, “zero-order” light propagates in a directionsubstantially parallel to that of the incident light, i.e., at asubstantially similar angle of incidence when the light is incident onan optical system along an optical axis of the optical system, and isalso referred to herein as “on-axis” light. For example, if the incidentlight is normally incident on the LCPG in a direction parallel to theoptical axis, “zero-order” or “on-axis” light would also propagatesubstantially normally with respect to the first polarization grating.In contrast, “non-zero-order light,” such as “first-order” light and/or“second-order light,” propagates in a direction that is not parallel tothe incident light nor the optical axis of the optical system. Inparticular, the second-order light propagates at greater angles than thefirst-order light relative to the angle of incidence. As such, first-and second-order light are collectively referred to herein as “off-axis”light.

LCPGs may be transparent, thin film, beam splitters that periodicallyalter the local polarization state and propagation direction of lighttraveling therethrough. Notably, during diffraction, the LCPG causes thepolarization handedness of the incident light to flip to its orthogonalcounterpart. Such characteristics are in contrast to conventionalpolarizers, which operate by permitting light of a first polarizationstate to travel therethrough, but absorbing light of an orthogonal,second polarization state.

A combination of two LCPGs may be aligned in parallel or in antiparallelconfigurations. Specifically, a “parallel” LCPG arrangement means therespective birefringence patterns of the two LCPGs have substantiallysimilar orientations. In contrast, an “antiparallel” polarizationgrating arrangement means one LCPG has a birefringence pattern that isinverted or rotated by about 180 degrees relative to that of the otherLCPG.

Non-mechanical beam steering can be achieved with an alternating stackof linear LCPGs and electro-optic half-wave retardance switches, someembodiments of which are described in the aforementioned U.S. Pat. No.8,982,313. Non-mechanical beam steering devices (also known as beamscanners) provide numerous benefits over traditional gimbaled mechanicalscanners due to their vastly reduced SWaP requirements and their abilityto perform random access scanning. To achieve non-mechanical beamscanning with LCPGs, a nematic or ferroelectric liquid crystal modulatorhaving an electronically controllable retardance is typically used asthe retardance switch, as mentioned above. In this case, the retardanceof the liquid crystal modulator is changed by applying a voltage toeither produce a half-wave of retardance or nearly zero retardancethrough the cell. Since a half-wave retarder changes the handedness ofcircularly polarized light while a cell with no retardance does notaffect the light's polarization, the incident light can be steered to aselected angle by controlling the handedness of circularly polarizedlight as it propagates through the LCPG stack. LCPGs have to date beendemonstrated with apertures up to 50 mm.

It would be desirable to have alternative LCPG devices with further SWaPand performance advantages.

SUMMARY OF THE DISCLOSURE

One aspect of the disclosure is a liquid crystal beam steering devicehaving a first polarization grating, a liquid crystal layer, a secondpolarization grating, a third polarization grating, an intermediateregion, a fourth polarization grating, and an aperture. The firstpolarization grating can be configured to direct incident light intofirst and second beams having different directions of propagation thanthat of the incident light. The first and second beams can havesubstantially orthogonal circular polarizations with respect to eachother. The liquid crystal layer can be configured to receive the firstand second beams from the first polarization grating. The liquid crystallayer can be switchable between first and second states for introducinga first and second retardance, respectively, to the first and secondbeams. The second polarization grating can be spaced apart from thefirst polarization grating by a distance D and can be configured toreceive the first and second beams from the liquid crystal layer. Thesecond polarization grating can also be configured to alter therespective directions of propagation of the first and second beamsaccording to the first or second retardance introduced to the first andsecond beams. The third polarization grating can be configured toreceive the first and second beams from the second polarization gratingand to further alter the respective directions of propagation thereof.The intermediate region can be configured to transmit the first andsecond beams from the third polarization grating therethrough. Thefourth polarization grating configured to receive the first and secondbeams from the intermediate region and to additionally alter therespective directions of propagation thereof to provide output light.The aperture can be configured to transmit a first portion of both thefirst and second beams from the fourth polarization grating when theliquid crystal layer is in the first state, and to transmit a secondportion of both the first and second beams from the fourth polarizationgrating therethrough when the liquid crystal layer is in the secondstate. The first portion can be greater than the second portion. Theintermediate region can have a thickness less than the distance D andcan be configured to separate the third and fourth polarization gratingsby the distance D.

Another aspect of the present disclose is a liquid crystal beam steeringdevice having a first polarization grating, a liquid crystal layer, asecond polarization rating, a third polarization grating, anintermediate region, a fourth polarization grating, and an aperture. Thefirst polarization grating can be configured to direct incident lightinto first and second beams having different directions of propagationthan that of the incident light. The first and second beams can havesubstantially orthogonal circular polarizations with respect to eachother. The liquid crystal layer can be configured to receive the firstand second beams from the first polarization grating. The liquid crystallayer can be switchable between first and second states for introducinga first and second retardance, respectively, to light travelingtherethrough. The second polarization grating can be spaced apart fromthe first polarization grating by a distance D1 and can be configured toreceive the first and second beams from the liquid crystal layer toalter the respective directions of propagation of the first and secondbeams. Such altering of the directions of the first and second beams canbe in response to each of the first and second states of the liquidcrystal layer. The third polarization grating can be configured toreceive the first and second beams from the second polarization gratingto further alter the respective directions of propagation thereof. Theintermediate region can have a thickness D2 and can be configured totransmit the first and second beams from the third polarization gratingtherethrough. The fourth polarization grating can be spaced apart fromthe third polarization grating by a distance D2 and can be configured toreceive the first and second beams from the third polarization gratingto additionally alter the respective directions of propagation thereofto provide output light that propagates in a direction substantiallyparallel to that of the first and second beams output from the secondpolarization grating. The aperture can be configured to block both firstand second beams when the liquid crystal layer is in the first state.The aperture can also be configured to transmit both first and secondbeams therethrough when the liquid crystal layer is in the second state.The incident light can be characterized by a wavelength λ. The liquidcrystal layer can exhibit a first refractive index n1(λ) at thewavelength λ. The intermediate region can exhibit a second refractiveindex n2(λ) at the wavelength λ. The distances D1 and D2 can be relatedby the equation D1*λ*n1(λ)=D2*λ*n2(λ).

Yet a further aspect of the disclosure can be described as a liquidcrystal beam steering device having a first polarization grating, aliquid crystal layer, a second polarization grating, a thirdpolarization rating, an intermediate region, a fourth polarizationgrating, and an aperture. The first polarization grating can beconfigured to direct incident light into first and second beams havingdifferent directions of propagation than that of the incident light. Thefirst and second beams can have substantially orthogonal circularpolarizations with respect to each other. The liquid crystal layer canbe configured to receive the first and second beams from the firstpolarization grating. The liquid crystal layer can be switchable betweenfirst and second states for introducing a first and second retardance,respectively, to light traveling therethrough. The second polarizationgrating can be spaced apart from the first polarization grating andconfigured to receive the first and second beams from the liquid crystallayer to alter the respective directions of propagation of the first andsecond beams in response to each of the first and second states of theliquid crystal layer. The third polarization grating can be configuredto receive the first and second beams from the second polarizationgrating to further alter the respective directions of propagationthereof. The intermediate region can be configured to transmit the firstand second beams from the third polarization grating therethrough whilemodifying the respective directions of propagation thereof. The fourthpolarization grating can be configured to receive the first and secondbeams from the intermediate region to additionally alter the respectivedirections of propagation thereof to provide output light thatpropagates in a direction substantially parallel to that of the firstand second beams output from the second polarization grating. Theaperture can be configured to block both first and second beams when theliquid crystal layer is in the first state, and to transmit both firstand second beams therethrough when the liquid crystal layer is in thesecond state.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates a LCPG beam steering device including an LCpolarization switch, in accordance with an embodiment of the presentdisclosure.

FIGS. 2 and 3 collectively illustrate the beam paths of lighttransmitted through the LCPG beam steering device of FIG. 1. FIG. 2illustrates the beam path when the LC polarization switch is in a firststate, and FIG. 3 illustrates the beam path when the LC polarizationswitch is in a second state.

FIG. 4 illustrates a LCPG beam steering device including an LCpolarization switch, in accordance with another embodiment of thepresent disclosure.

FIG. 5 illustrates a LCPG beam steering device including an LCpolarization switch, in accordance with yet another embodiment of thepresent disclosure,

FIG. 6 illustrates a LCPG beam steering device including an LCpolarization switch, in accordance with yet another embodiment of thepresent disclosure.

FIG. 7 illustrates a LCPG beam steering device including an LCpolarization switch and trim retarders, in accordance with anotherembodiment of the present disclosure.

FIG. 8 illustrates a retardance versus voltage plot for an LC switchsystem that can be implemented in any of the herein-describedembodiments.

FIG. 9 shows another retardance versus voltage plot for a switchablephase modification system having different parameters than the systemunderlying FIG. 8.

FIG. 10 shows another retardance versus voltage plot, but where thevoltages for the first and second states are both positive and thus theLC switch can be said to be on for both states.

FIG. 11 shows another retardance versus voltage plot showing the resultsof using a higher retardance LC switch than that of FIG. 10.

FIG. 12 shows a retardance versus voltage plot for four different statesof the LC switch.

FIG. 13 illustrates a plot showing retardance as a function of a voltageapplied to a LC switch in an LCPG system; and

FIG. 14 illustrates an LCPG system having an AC bias and optionalfeedback for controlling the AC bias.

FIG. 15 illustrates a method of operating an LCPG system according toone embodiment of this disclosure.

DETAILED DESCRIPTION OF EMBODIMENTS OF THE DISCLOSURE

The present disclosure is described more fully hereinafter withreference to the accompanying drawings, in which embodiments of thedisclosure are shown. This disclosure may, however, be embodied in manydifferent forms and should not be construed as limited to theembodiments set forth herein. Rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the disclosure to those skilled in the art. In thedrawings, the size and relative sizes of layers and regions may beexaggerated for clarity. Like numbers refer to like elements throughoutthe specification.

It will be understood that, although the terms first, second, third etc.may be used herein to describe various elements, components, regions,layers and/or sections, these elements, components, regions, layersand/or sections should not be limited by these terms. These terms areonly used to distinguish one element, component, region, layer orsection from another region, layer or section. Thus, a first element,component, region, layer or section discussed below could be termed asecond element, component, region, layer or section without departingfrom the teachings of the present disclosure.

Spatially relative terms, such as “beneath,” “below,” “lower,” “under,”“above,” “upper,” and the like, may be used herein for ease ofdescription to describe one element or feature's relationship to anotherelement(s) or feature(s) as illustrated in the figures. It will beunderstood that the spatially relative terms are intended to encompassdifferent orientations of the device in use or operation in addition tothe orientation depicted in the figures. For example, if the device inthe figures is turned over, elements described as “below” or “beneath”or “under” other elements or features would then be oriented “above” theother elements or features. Thus, the exemplary terms “below” and“under” can encompass both an orientation of above and below. The devicemay be otherwise oriented (rotated 90 degrees or at other orientations)and the spatially relative descriptors used herein interpretedaccordingly. In addition, it will also be understood that when a layeris referred to as being “between” two layers, it can be the only layerbetween the two layers, or one or more intervening layers may also bepresent.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the disclosure.As used herein, the singular forms “a,” “an,” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprises”and/or “comprising,” when used in this specification, specify thepresence of stated features, integers, steps, operations, elements,and/or components, but do not preclude the presence or addition of oneor more other features, integers, steps, operations, elements,components, and/or groups thereof. As used herein, the term “and/or”includes any and all combinations of one or more of the associatedlisted items, and may be abbreviated as “/”.

It will be understood that when an element or layer is referred to asbeing “on,” “connected to,” “coupled to,” or “adjacent to” anotherelement or layer, it can be directly on, connected, coupled, or adjacentto the other element or layer, or intervening elements or layers may bepresent. In contrast, when an element is referred to as being “directlyon,” “directly connected to,” “directly coupled to,” or “immediatelyadjacent to” another element or layer, there are no intervening elementsor layers present. Likewise, when light is received or provided “from”one element, it can be received or provided directly from that elementor from an intervening element. On the other hand, when light isreceived or provided “directly from” one element, there are nointervening elements present.

Embodiments of the disclosure are described herein with reference tocross-section illustrations that are schematic illustrations ofidealized embodiments (and intermediate structures) of the disclosure.As such, variations from the shapes of the illustrations as a result,for example, of manufacturing techniques and/or tolerances, are to beexpected. Thus, embodiments of the disclosure should not be construed aslimited to the particular shapes of regions illustrated herein but areto include deviations in shapes that result, for example, frommanufacturing. Accordingly, the regions illustrated in the figures areschematic in nature and their shapes are not intended to illustrate theactual shape of a region of a device and are not intended to limit thescope of the disclosure.

Unless otherwise defined, all terms (including technical and scientificterms) used herein have the same meaning as commonly understood by oneof ordinary skill in the art to which this disclosure belongs. It willbe further understood that terms, such as those defined in commonly useddictionaries, should be interpreted as having a meaning that isconsistent with their meaning in the context of the relevant art and/orthe present specification and will not be interpreted in an idealized oroverly formal sense unless expressly so defined herein.

It will be understood by those having skill in the art that, as usedherein, a “transmissive” or “transparent” substrate may allow at leastsome of the incident light to pass therethrough. Accordingly, thetransparent substrate may be, for example, formed of glass, sapphire, orother materials.

Embodiments of the present disclosure are described herein withreference to the accompanying figures. Referring first to FIG. 1, a LCbeam steering device 100 is described. LC beam steering device 100includes a first LCPG 102, which is supported on a first substrate 104.A second LCPG 112, supported on a second substrate 114, is spaced apartfrom first LCPG 102 by a distance D1. First and second LCPGs 102 and 112may be formed, for example, using methods described in U.S. Pat. No.7,196,758 to Crawford et al., which is incorporated herein in itsentirety by reference.

As shown in FIG. 1, first and second substrates 104 and 114,respectively, are configured to contain therebetween a liquid crystal(“LC”) switch 120. LC switch 120 includes liquid crystal molecules thatare configured to be switched between first and second states, inresponse to voltages applied thereacross. The surfaces of first andsecond substrates 104 and 114 that contain the LC switch are treatedwith alignment layers and transparent conductive layers (not shown) soas to align the LC switch in a desired configuration as well as to allowthe application of a voltage across the LC switch. For example, thealignment layer may be a commercial polyimide coating, such as NissanChemical Industries SE-7492, and the transparent conductive layers maybe formed, for example, of standard coatings such as Indium Tin Oxide(“ITO”) or Indium Molybdenum Oxide (“IMO”). In some embodiments, an LCswitch can also be referred to as a liquid crystal layer.

LC beam steering device 100 further includes a third LCPG 132, which issupported on a third substrate 134. Third LCPG 132 is separated fromsecond LCPG 112 by a distance d. In some embodiments, the distanced maybe the thickness of an optical adhesive or index-matching layer (notshown) used to bond together second and third LCPGs 112 and 132,respectively. Alternatively, second LCPG 112 and third LCPG 132 may beplaced in direct contact with each other. In such embodiments, thedistance d is much smaller than the distance D1 shown in FIG. 1.

Still further, LC beam steering device 100 includes a fourth LCPG 142,which is supported on a fourth substrate 134 and spaced apart from thirdLCPG 132 and its supporting third substrate 134 by a distance D2. Aspace 150 (or intermediate region) defined between third substrate 134and fourth substrate 144 may be filled, for example, with a materialsuch as an index-matching fluid, optical adhesive, or air.

It should be emphasized that various components in the figures describedherein are not drawn to scale. For example, in FIG. 1 and subsequentfigures, the various substrates may have thicknesses ranging from 50 to2000 microns, or thicker, depending on the material used and desiredcharacteristics. Also, LC switch 120 and space 150 may have a thickness,for example, in the range of 1 to 10 microns, depending on the materialcharacteristics of the material used therein. In certain cases, the LCswitch thickness may be less than one micron, or more than 10 microns,depending on the optoelectronic characteristics of the LC material.

LC beam steering device 100 additionally includes an aperture 160. Forexample, the aperture may be an explicit aperture in a piece of opaquematerial, as shown in FIG. 1. Alternatively, the aperture may be animplicit aperture created by the finite extent of another component, notshown, such as a lens or optical fiber, or by the acceptance aperture ofa subsequent optical system. The components of LC beam steering device100 in this exemplary embodiment are aligned with respect to an opticalaxis 170.

First, second, third, and fourth LCPGs 102, 112, 132 and 142,respectively, may provide diffraction properties such as at least onediffracted orders (such as +1 or −1 order), substantially orthogonalcircular polarizations of the non-zero orders, and/or highlypolarization-sensitive non-zero-orders, which may be linearlyproportional to the Stokes parameter of the LCPGs. For example, theLCPGs may be polymerized LC films including anisotropic periodicmolecular structures with birefringence patterns configured to diffractlight incident thereon with a diffraction efficiency of 50% or greater.Each one of first, second, third, and fourth LCPGs 102, 112, 132 and142, respectively, may include multiple layers having periodic localanisotropy patterns that are offset relative to one another to define aphase modification therebetween and/or rotated by a twist angle overrespective thicknesses thereof. Additionally, one or more of suchmultiple layers may be an actively switchable liquid crystal layer suchthat the LCPG acts as a switchable liquid crystal polarization grating.

First, second, third, and fourth LCPGs 102, 112, 132 and 142,respectively, may be identical in type, thickness, periodicity, and/ormolecular orientation, or one or more of the LCPGs may have acharacteristic distinct from the other LCPGs in LC beam steering device100. Furthermore, first, second, third, and fourth LCPGs 102, 112, 132and 142, respectively, may be arranged in parallel or antiparallelorientation with respect to each other.

The operation of an exemplary embodiment of LC beam steering device 100is illustrated in FIGS. 2 and 3. In FIG. 2, LC switch 120′ is in a firststate, in which a first retardance is introduced to light travelingthrough the LC switch (retardance can also be referred to as a phasemodification between polarizations of light passing through the LCswitch 120). In FIG. 3, LC switch 120″ is in a second state, in which asecond retardance is introduced to light traveling through the LCswitch. In the exemplary embodiment illustrated in FIGS. 2 and 3, firstand second LCPGs 102 and 112, respectively, are arranged in a parallelorientation (as indicated by arrows 201 and 203), and third and fourthLCPGs 132 and 142, respectively, are also arranged in a parallelorientation (as indicated by arrows 205 and 207). In the illustratedexample, the orientation of the pair formed by first and second LCPGs102 and 112, respectively, is antiparallel to the orientation of thepair formed by third and fourth LCPGs 132 and 142, respectively. It isassumed throughout the description of the exemplary embodiment in FIGS.2 and 3 that the diffractive properties, such as the grating pitch andthickness, of first, second, third, and fourth LCPGs 102, 112, 132, and142, respectively, are essentially identical. However, in some cases,the diffractive properties of the first, second, third, and fourth LCPGs102, 112, 132, and 142 can be substantially similar. While thediffractive properties can be the same or similar, the orientations ofone or more of the LCPGs can be antiparallel.

As shown in FIG. 2, a light beam 200′ is incident on first LCPG 102along optical axis 170. Light beam 200′ is characterized by a firstpolarization state. For instance, light beam 200′ may be characterizedby right-hand circular polarization. However, in other embodiments, theincident light beam 120 can be unpolarized, elliptically polarized, orhave some other polarization state. When any but circularly-polarizedlight is incident on the first LCPG 102, two beams will emerge from thefirst LCPG 102.

Upon transmission therethrough, first LCPG 102 diffracts light beam 200′by an angle A1′ with respect to optical axis 170, and the polarizationstate of light beam 200′ is flipped to left-hand circular polarization.LC switch 120′ is in a first state, which introduces a first retardanceto light beam 200′ upon transmission therethrough. For example, thefirst retardance may be a full-wave retardance; in this case, light beam200′ remains left-hand circularly polarized when incident on second LCPG112. However, any multiple of a full wave, or λ can be imparted by theLC switch 120′ in this first state. Said another way, in the firststate, the LC switch 120′, along with any trim retarders, can impart aretardance of nλ, where n can be selected from the set of integers aswell as 0. Assuming the first state imparts a multiple of a full-waveretardance to the beam 200′, and since LCPG 112 is oriented in parallelto first LCPG 102, second LCPG 112 then diffracts light beam 200′ topropagate substantially parallel to optical axis 170, while flipping thehandedness of the polarization state such that light beam 200′ emergingfrom second LCPG 112 is again right-circularly polarized.

Light beam 200′ is then incident on third LCPG 132. As third LCPG 132 isoriented in an antiparallel manner with respect to first and secondLCPGs 102 and 112, light beam 200′ is diffracted at an angle −A1′. Thepolarization state of light beam 200′ is once again flipped such thatlight beam 200′ emerging from third LCPG 132 is again left-circularlypolarized.

Light beam 200′ is subsequently transmitted through space 150 with itsleft-circular polarization intact until it is incident on fourth LCPG142. Fourth LCPG 142, being parallel in orientation to third LCPG 132,diffracts light beam 200′ back in alignment with optical axis 170 andwith right-hand circular polarization such that light beam 200′ issubsequently transmitted through aperture 160.

Turning now to FIG. 3, a light beam 200″ with right-hand circularpolarization is incident on first LCPG 102. First LCPG 102 diffractslight beam 200″ again at an angle A1′ and flips the polarization to aleft-hand circular polarization. This time, LC switch 120″ is in asecond state such that light transmitted therethrough experiences asecond retardance. The second retardance may be, for instance, ahalf-wave retardance; in this case, light beam 200″ experiences ahalf-wave retardance during transmission through LC switch 120″ suchthat light beam 200″ is characterized by a right-hand circularpolarization and consequently diffracted by second LCPG 112 at an angleA2″, which is larger than angle A1′. The handedness of the polarizationof light beam 200″ again is flipped upon diffraction by second LCPG 112such that the light beam emerging from second LCPG 112 is left-handcircularly polarized. In other embodiments, the LC switch 120″ in thissecond state, along with any trim retarders, can impart any m+λ/2retardance to the light beam 200″, where m can be selected from the setof integers as well as 0.

Since third LCPG 132 is in an antiparallel orientation with respect tofirst and second LCPGs 102 and 112, respectively, light beam 200″ isfurther diffracted to an angle A3″, which is larger than angle A2″, upontransmission through third LCPG 132. Light beam 200″ then propagatesthrough space 150 with right-hand circular polarization, then is furtherdiffracted by fourth LCPG 142 into an angle A4″, which is still largerthan angle A3″, with left-hand circular polarization. Finally, lightbeam 200″ emerging from fourth LCPG 142 can be blocked by aperture 160.If, for instance, the aperture is instead an implicit aperture, aspreviously discussed, light beam 200″ does not enter the opticalcomponent or system located further along optical axis 170 from fourthLCPG 142.

For the exemplary embodiment illustrated in FIGS. 1, 2, and 3, in otherwords, light beam 200 is transmitted through aperture 160 when LC switch120 is in a first state, and light beam 200 is blocked by aperture 160when LC switch is in a second state. In other embodiments, the system100 can be arranged such that switching the LC switch 120 results in apartial transmission or blocking of the light beam 200. For instance,when the LC switch 120 is in a first state, the light beam 200 may be atleast partially transmitted through the aperture 160, while the lightbeam 200 may be at least partially blocked by the aperture 160 when theLC switch 120 is in a second state. As a further example, the LC switch120 in a first state, either in combination with one or more trimretarders, may impart other than a multiple of a half wave of retardanceto the light beam 200′. For instance, in a first state (or an “off”state) the LC switch 120′ plus one or more trim retarders or otherretarding mechanisms can apply a retardance or phase modification of

$\frac{\lambda}{10}$

waves to the light beam 200′ between LCPG 102 and LCPG 112. The resultwould be a slight change in the polarization of the light beam 200 thatcauses an output light beam 200′ to split into two components ofdifferent powers, one following the path shown in FIG. 2, and onefollowing the path shown in FIG. 3. In this way, the first state of theLC switch 120′ can result in some attenuation of the light beam 200′,although this attenuation is not equal to nor comparable to theattenuation seen when the LC switch 120′ is in a second or “on” state.

Similarly, a second state of the LC switch, either alone or incombination with one or more trim retarders, may impart other than amultiple of a quarter wave retardance to the light beam 200″. In thisway, the light beam 200″ changes polarization between LCPG 102 and LCPG112 (sees a phase modification between polarizations or a retardance),but does not undergo a full 90° or quarter-wave change in polarization.Rather, the retardance can be close to a 90° or quarter-wave retardance,but not equal thereto. The result, is that the second state of the LCswitch 120″ results in less than full attenuation at the aperture 160.In other words, if the system 100″ is spaced and sized appropriately,the second state of the LC switch 120″ can result in some portion of thelight beam 200″ passing through the aperture 160.

In another embodiment, a first state of the LC switch 120′ results in anentirety of the light beam 200′ passing through the aperture 160, whilea second state of the LC switch 120″ results in some, but not all of thelight beam 200″, passing through the aperture 160. A third state of theLC switch 120, between the first and second states of the LC switch 120,results in some portion of the light beam 200 passing through theaperture 160, where this portion is larger than that transmitted giventhe first state of the LC switch 120′, yet smaller than that transmittedgiven the second state of the LC switch 120″. Third, fourth, fifth, etc.states of the LC switch 120 can also be implemented in order to increasethe selectivity of transmission amounts through the aperture 160.

These examples show that the LC switch 120 may have intermediate statesthat result in less than a maximum contrast between first and secondstates, or on and off states. Alternatively, there may be more than twostates, and thereby variable attenuation can be achieved.

In one embodiment, such variable attenuation can be accomplished throughan AC bias applied to the LC switch 120, where the AC bias is configuredto apply a variety of AC biases to switch the LC switch 120 betweenvarious states between and including the first and second states.Different biases result in a different level of alignment within the LCswitch 120 and hence different amounts of retardance can be imparted tothe light beam 120. In other words, the attenuation of the light beam120 can be a function of the AC bias applied to the LC switch 120. In anembodiment, the AC bias applied in either the first or second state is0V. In another embodiment, the AC bias for both the first and secondstate can be greater than 0V.

Returning briefly to FIG. 1, it should be noted that the distances D1and D2 are strategically determined to achieve the appropriateperformance by LC beam steering device 100. In one example, assuming thematerial composition and thicknesses of first, second, third, and fourthsubstrates 104, 114, 134, and 144 are essentially identical (i.e.,essentially identical indices of refraction), then distances D1 and D2may be set to be equal if the refractive indices of the materialscomprising LC switch 120 and space 150 are similar. If the thicknessesof LC switch 120 and space 150 are small compared to the thickness ofthe substrates, as is likely in a practical implementation, theprecision required of the refractive index match between LC switch 120and any material contained within space 150 may be reduced. Such asetting may be achieved, for example, by using the same spacerarrangement (not shown) in setting the thicknesses of both LC switch 120and space 150. Suitable spacer arrangements may include, for example,the use of spacer beads or spacer rods suspended in optical adhesive.Further, the above-mentioned relationship of D1 and D2 assumes thatbonding layers such as glues, have a negligible effect on beam steering.Where such layers do have a noticeable effect on beam steering, theirinfluence can be factored into the relationship between D1 and D2.

In a further refined calculation, the angles of deviation of thegratings and the propagation through multiple layers are “balanced” sothat the light beam, in the transmitting state, is correctly returned tothe optical axis. This calculation is performed by tracing a ray throughthe stack and evaluating the result of refraction at layer boundariesusing Snell's law. Such a holistic, optical system view of the LC beamsteering device allows the inclusion of manufacturability considerationsinto the device design, thereby greatly increasing the configurationflexibility of the entire system. We have recognized that factoringmanufacturability and LC and LCPG material issues into the LC beamsteering device design is essential to the implementation of a practicaland consistently manufacturable devices with superior SWaPcharacteristics.

As an example, setting D1=D2, as shown in FIGS. 1-3, may be useful suchthat, light beam 200 may be inserted into LC beam steering device 100along optical axis 170 and subsequently exit through aperture 160 againalong optical axis 170 when LC switch 120 is in a first state, as shownin FIG. 2. This choice of setting D1=D2 assumes that the refractiveindex of substrates 104, 114, 134, and 144 as well as the refractiveindices of LC switch 120 and any material contained within space 150 aresubstantially the same at the wavelength of interest or, if a layer hasa significantly different refractive index, its thickness issufficiently small so as to not displace the beam too far (e.g., bondinglayers such as glues or optical fillers). Additionally, manufacturingtolerances can cause small changes in D1 and D2 that do not causeuntenable beam displacement. The allowed tolerances can depend on beamdiameter and other factors, but, for instance, the inventors found thatgiven 700 μm thick substrates 104, 114, 134, 144 having manufacturingtolerances of +/−50 μm each, the influence on beam displacement of thesesmall divergences from D1=D2 were acceptable. Given smaller beamdiameters, smaller tolerances may be preferred. Thus, one could say thatD1=D2 within the bounds of manufacturing tolerances for a givenapplication (e.g., given a certain beam diameter among others).

Additionally, setting D1=D2, in the system 100, enables a single typeLCPG to be used for all four LCPGs 102, 112, 132, 142. In other words,from a manufacturing standpoint, setting D1=D2 enables a single type ofLCPG having singular parameters to be used for all four of the LCPGs102, 112, 132, 142 in the system 100. For instance, the samemanufacturing setup can be used for all four LCPGs 102, 112, 132, 142(e.g., two or more of the LCPGs 102, 112, 132, 142 can be formed on thesame substrate). Alternatively, a large LCPG can be made, and many smallgratings can be formed therefrom by cutting the large LCPG with a dicingsaw or scribe-and-break system. The use of four identical, ornearly-identical, LCPGs provides cost and manufacturing advantages overthree-grating systems, where each LCPG would need to be different.

Another advantage of the four-grating system 100 is enhanced contrastratio or dynamic range as compared to three-grating systems. Komanduriet al. (A High Throughput Liquid Crystal Light Shutter for UnpolarizedLight Using Polymer Polarization Gratings; Acquisition, Tracking, andLaser Systems Technologies XXV, Proc. of SPIE Vol. 8052, 2011) discuss athree-grating system for use in displays and describes contrast ratiosas high as 230:1. The herein disclosed four-grating system is able toachieve much higher contrast ratios including those greater than 1000:1.

Alternatively, D1 and D2 may be set to be purposefully unequal such thatlight beam 200 emerges at an off-axis angle or off-set from optical axis170. Such embodiments may be useful in certain system configurationsthat require off-axis inputs and/or outputs.

FIG. 4 shows another variation in which first and second LCPGs 102 and112, respectively, may be a matched pair sharing substantially similarperiodic birefringence patterns, while third and fourth LCPGs 132 and142, respectively, are another matched pair sharing substantiallysimilar periodic birefringence patterns with respect to each other,although different from the periodic birefringence patterns of first andsecond LCPGs 102 and 112, respectively. For a light beam 401 incident onLC beam steering device 400 along optical axis 170, the correspondingD1′ and D2′ values are related by the beam propagation angles B1 and B2so that:

D1′*sin(B1)+D2′*sin(B2)=0  Eq. (1)

One of skill in the art will recognize that optical tolerances of up to10% are common, and therefore, tolerances of up to around 10% in D1′ andD2′ are acceptable without departing from Eq. (1). For instance, athickness of the liquid crystal switch (e.g., 3 μm) 420 is unlikely tohave a noticeable effect on Eq. (1) in many use cases. For instance,where the substrates 104, 114, 134, 144 are around 700 μm in thickness,nominal changes in thickness (e.g., +/−30 μm), for instance, that of LCswitch 420, the space 150, and the thicknesses of the LCPGs 102, 112,132, 142, are unlikely to have a noticeable effect on Eq. (1).Tolerances of D1′ and D2′ may depend on incident beam diameter: widerbeams may suggest greater tolerance, while narrower beams may suggestlesser tolerance. In other words, various manufacturing tolerances onD1′ and D2′ are envisioned, and those of skill in the art will be ableto apply Eq. (1) given acceptable tolerances for a given application.The angles shown in this figure illustrate the situation in which therefractive indices of LC switch 420 and any material contained withinspace 150 are similar to that of first, second, third, and fourthsubstrates 104, 114, 134, and 144, respectively.

FIG. 5 illustrates another exemplary embodiment, in which thickness D52of third substrate 534, thickness D53 of space 550, and thickness D54 offourth substrate 544 are set to be significantly different from eachother and distance D1. Diffractive angles are traced through media ofdifferent refractive indices. In this example, a light beam 501 is firstdeflected at first LCPG 102 up to an angle C1, at which it propagatesfor distance D1. Upon encountering second LCPG 112, light beam 501 isthen redirected to a direction substantially parallel to the opticalaxis. Light beam 501 then propagates for distance d until it encountersa third LCPG 532, at which point light beam 501 is then directed down toan angle C2 for propagation distance D52 through a third substrate 534.In this example, the material in a space 550 is assumed to have a lowerrefractive index than that of substrate 534, so light beam 501 deviatesfurther down to an angle C3 as it propagates for distance D53 throughspace 550. Light beam 501 is then incident on a fourth substrate 544which causes a refraction to an angle C4. It may be noted that, if thirdsubstrate 534 and fourth substrate 544 are formed of differentmaterials, then angles C2 and C4 would not be equal. Finally, a fourthLCPG 542, causes a deflection of the beam back parallel to optical axis170. Snell's law may be used to determine the angle of propagation afterthe transition into a new medium such as from substrate 534 to space550.

In the situation which is shown in FIG. 5, which has the exit aperturepositioned on-axis with respect to optical axis 170, the deflectionangles and distances should be chosen so that:

D1*sin(C1)+D52*sin(C2)+D53*sin(C3)+D54*sin(C4)=0  Eq. (2)

In this way, a variety of material and thickness configurations may beaccommodated to achieve an effective and practical device design. Again,manufacturing tolerances appropriate for the use case are envisionedrelative to the distances specified in Eq. (2).

In yet another variation the functions of the third and fourthpolarization gratings, above, may be combined into a single LCPG, asillustrated in FIG. 6. In this exemplary embodiment, a first LCPG 602deflects a light beam 601 to an angle of A61. The beam then passesthrough a LC switch 620, which may “flip” the polarization state to theorthogonal handedness, depending on the state of the liquid crystalmaterial contained therein. A second LCPG 612 may be configured suchthat light beam 601 may be deflected back towards optical axis 170.Finally, light beam 601 is brought back into alignment with the opticalaxis by a third LCPG 642. In this configuration, the diffractiveproperties of first, second, and third LCPGs 602, 612, and 642,respectively, are chosen so that the resulting deflection angles arerelated by the following equation:

D61*sin(A61)+D62*sin(A62)=0  Eq. (3)

Note that the magnitude of the deflection angle effected by second LCPG612 is equal to the sum of the magnitudes of A61 and A62, and thedeflection effected by third LCPG 642 is equal in magnitude to A62.Again, manufacturing tolerances appropriate for the use case areenvisioned relative to the distances specified in Eq. (3).

In yet another variation, the LC switch may be switched between twostates that are separated by approximately a half wave of retardance,with the values of retardance chosen for reasons of LC switching speed,convenience of cell assembly, drive voltage range, or a combination ofthese factors. For example, the LC cell could be an untwistedelectrically-controlled birefringence (“ECB”) cell configured to switchbetween a high-voltage state, with a retardance of less than aquarter-wave, and a low-voltage state, with a retardance approximatelyone half-wave greater. One or more retarders, external to the LC cell,may be added to “trim” the effective retardance of the ECB cell suchthat, in one of the LC cell's states, the light's polarization issubstantially unaltered and, in the other state, the light'spolarization is changed to the orthogonal circular polarization state.

An example of an ECB cell implementation of an LC beam steering deviceis shown in FIG. 7. FIG. 7 shows a LC beam steering device 700 includingfirst, second, third, and fourth LCPGs 102, 112, 132, and 142,respectively, supported on first, second, third, and fourth substrates104, 114, 134, and 144, respectively. Rather than having an LC switchsupported between first and second substrates 104 and 114, respectively,a combination of elements are supported between first and secondsubstrates 104 and 114. As shown in FIG. 7, a first trim retarder 715and an optional second trim retarder 725 is/are placed on either side ofan ECB cell 735. ECB cell arrangement includes fifth and sixthsubstrates 754 and 764, respectively, supporting a liquid crystal layer740 therebetween.

In an exemplary embodiment, first trim retarder 715 or the combinationof first and second trim retarders 715 and 725, respectively, may bechosen to compensate for any residual retardance of ECB cell 735 so thecombination of ECB cell 735, in one state, and trim retarder(s) leavesthe incident light's polarization substantially unaffected. Forinstance, ECB cell 735 may be configured and driven to a high or firstvoltage for one if its switched states. If, in this high or firstvoltage state, the residual retardance of ECB cell 735 is 80 nm, as anexample, then first trim retarder 715 may be selected to exhibit aretardance of 80 nm at the wavelength of interest and second trimretarder 725 may be eliminated from LC beam steering device 700. Firsttrim retarder 715 may be oriented, for example, with its in-planeslow-axis at 90 degrees to an in-plane slow-axis of ECB cell 735. Thischoice of orientation results in the residual retardance of ECB cell 735and the retardance of first trim retarder 715 cancelling each other suchthat the polarization state of the light transmitted through thecombination of first trim retarder 715 and ECB cell 735, in the high orfirst voltage state, is essentially unaltered.

First and second trim retarders 715 and 725 may be formed, for instance,by combining a plurality of retarders in order to obtain the requiredretardance value to cancel out the residual retardance of the particularECB cell selected to be used within the system. Continuing the previousexample, if it proves inconvenient to purchase or make 80 nm retardermaterial, it may be more convenient to acquire retarders of other valuesand combine them appropriately. For instance, a retarder of 350 nm couldbe crossed with (i.e., oriented at 90 degrees to) a retarder of 270 nmto yield a composite retarder of 80 nm. Similarly, a retarder of 30 nmcould be additively combined with a retarder of 50 nm to achieve acomposite retarder of 80 nm, by combining them with their slow axes inparallel. If the trim retarder is made from two or more separate parts,these component parts may be placed on either or both sides of ECB cell735 as first and second trim retarders 715 and 725, respectively.

Alternatively, it may be convenient to use a trim retarder arrangementthat modifies the combined retardance of the assembly to a value thatdoes change the polarization state of the incident light when ECB cell735 is in the high or first voltage state. For instance, if ECB cell 735exhibits a residual retardance of 80 nm in the high or first voltagestate and the wavelength of interest is 500 nm, then it may beconvenient to trim the cell from 80 nm to 250 nm by using a first and/orsecond retarder 715 and 725, respectively, with an effective trimretardance of 170 nm. The value of the low or second voltage may then bechosen such that ECB cell 735 exhibits a retardance of approximately 330nm in the low or second voltage state, so that the retardance of thecombined ECB cell and trim retarder arrangement would be approximately500 nm (i.e., one wave at the wavelength of interest). In this case, thehigh voltage state (or first state) of ECB cell 735 would flip thepolarization state of the incident light, and the low voltage state (orsecond state) would leave the polarization state of the incident lightsubstantially unchanged, thus resulting in different beam propagationpaths through LC beam steering device 700.

There are a variety of ways to arrange the combination of ECB cell andtrim retarder(s) to provide the required switching function. Theappropriate switching function may be achieved with a configuration thatprovides a combined retardance approximately equal to an even number ofhalf-waves in one state of the ECB cell, and an odd number of half-wavesin the other state of the ECB cell. The choice of components will dependon the relative importance of engineering factors, such as switchingspeed, available voltage, temperature range requirements, manufacturingcost and availability of retarders at the desired retardance values.

In another variation, ECB cell 735 in FIG. 7, may be replaced with a picell (also known as an Optically Compensated Bend cell) (See, forexample, P. J. Bos and K. R. Koehler-Beran, Mol. Cryst. Liq. Cryst. 113,329 (1984)). This alternative may be a good choice of LC configurationfor applications requiring sub-millisecond switching speed, and is anexample of an LC cell configuration that works well with a trimretarder. Although a pi cell may be constructed and driven between ahalf-wave and a full wave of retardance, one can obtain fasterperformance by driving between two states of lower retardance.Consequently, by combining a pi cell with one or more trim retarders,faster switching speeds may be obtained.

In yet a further variation, ECB cell 735 in FIG. 7, may be replaced withone or more ferroelectric liquid crystals (FLCs). This alternative maybe a good choice of LC configuration for applications requiring greaterspeed than those using a pi cell. Each FLC is typically arranged tocomprise a quarter-wave of retardance. The orientation of the FLC can bein-plane, oriented at an angle that depends on the state of an appliedvoltage to the FLC. The FLC material is selected to switch between twostates separated by approximately 45 degrees. Thus, when two FLCs areused in combination and two states of a voltage are applied to the pairof FLCs, the combined FLCs can either add, to apply a half-wave ofretardance if they are parallel, or subtract, to apply no retardance ifthey are oriented 90 degrees to each other.

While FLCs have been around for some time and were seen as having greatpotential for use in displays, they tend to produce patchy images whenused in displays due to difficulties in achieving uniform alignment andtheir less-than-desired response to analogue switching inputs. Thus,FLCs are considered to have inherent disadvantages that make themunlikely contenders for switching applications. Yet, the inventorsrecognized that a much greater attenuation tolerance could be affordedin certain applications, such as where a single beam is being directedthrough an aperture. Unexpectedly, FLCs have application here despitetheir inherent disadvantages for switching applications.

Turning back now to switching of the LC switch, a better understandingmay be possible via reference to the following equations and FIGS. 8-10.Equation 4 represents a retardance imparted by any combination of one ormore LC switches or LC layers, along with any one or more trim retardersin a first state (where the LC switch is on), where n is selected fromthe set of integers as well as 0 (i.e., the retardance of an even numberof half wave retarders). Equation 5 represents a phase modificationimparted by any combination of one or more LC switches or LC layers,along with any one or more trim retarders in a second state (where theLC switch is off), where m is selected from the set of integers as wellas 0 (i.e., the retardance of an odd number of half wave retarders). Forinstance, an LC switch could impart a 2λ retardance in the first state,where n=2 or a λ retardance in the first state, where n=1.

$\begin{matrix}{n\; \lambda} & {{Eq}.\mspace{14mu} (4)} \\{m + \frac{\lambda}{2}} & {{Eq}.\mspace{14mu} (5)}\end{matrix}$

FIGS. 8-10 show some different scenarios where equations 4 and 5 areused to explain or design a system of one or more LC switches, or one ormore LC switches in combination with one or more trim retarders. In eachfigure a cumulative retardance of the one or more LC switches andoptional one or more trim retarders is shown on the y-axis, while an ACvoltage applied to the LC switch at a first state, V₁, and at a secondstate, V₂, are shown on the x-axis.

FIG. 8 illustrates a retardance versus voltage plot for an LC switchsystem that can be implemented in any of the above-describedembodiments. The spline curve represents a retardance imparted by one ormore LC switches, or one or more LC switches in combination with one ormore trim retarders, for different AC voltages applied to the one ormore LC switches. For simplicity, the one or more LC switches, or one ormore LC switches in combination with one or more trim retarders will bereferred to as a “switchable retardance system.” One will recognize thatin practice the voltage applied in a multi-LC-switch configuration maybe more complicated than that shown since different voltages may beapplied to each LC switch. However, for purposes of these illustrations,one can assume that the same voltage is applied to the one or more LCswitches.

In a first state, a voltage V₁ is applied to the switchable retardancesystem, and a relative phase modification of 0 is imparted to lightbeams passing through the switchable retardance system. In a secondstate, no voltage is applied to the switchable retardance system, and ahalf wave, or

$\frac{\lambda}{2},$

of retardance is imparted to any light beams passing through theswitchable retardance system. In the second state, the retardance,

$\frac{\lambda}{2},$

is the inherent or default retardance of the LC switch. A thickness andtype of the one or more components in the switchable retardance systemcan dictate the shape of the curve and the spline's intersections withthe x and y axes (although in some cases the curve does not intersectthe x-axis due to residual retardance).

Typical LC switches are unable to impart a retardance of 0 due toresidual retardance. Even at very large voltages, the retardance curvefor most LC switches does not intersect the x-axis. Instead, an infinitevoltage is required to apply zero retardance, and such a voltage is notpractical. FIG. 13 shows the voltage versus retardance plot for such anLC switch.

One way to achieve a retardance of 0, as shown for the first state inFIG. 8, is to form a switchable retardance system comprising an LCswitch and a trim retarder, where the trim retarder shifts theretardance curve of the LC switch seen in FIG. 13. The LC switchthickness can then be increased such that the spline curve'sintersection of the y-axis still occurs at

$\frac{\lambda}{2},$

as seen in FIG. 8. While this is one switchable retardance system thatcan achieve the plot in FIG. 8, this example shows that a variety ofother switchable retardance systems can also achieve the plot in FIG. 8.

Further, FIG. 8 shows that switching between two applied AC voltagesenables a switchable retardance system to impart either a half wave ofretardance or no retardance, as shown in FIGS. 2-3. One can also seethat by applying different voltages between V₁ and V₂ other retardancesbetween 0 and a half-wave of retardance can be imparted. Thus, statesother than the first and second state are also possible and therefore avariable tuning of the beam steering and attenuation is possible (seeFIGS. 10-12).

FIG. 9 shows another retardance versus voltage plot for a switchableretardance system having different parameters than the system underlyingFIG. 8. One can see that in the second state, the retardance is equal tothree half waves,

${\frac{3}{2}\lambda},$

which is effectively a half-wave retardance. In the first state, thereis no retardance. As can be seen, the systems underlying FIGS. 8 and 9impart the same effective retardance in the first and second states.

FIG. 10 shows another retardance versus voltage plot, but where thevoltages for the first and second states are both positive and thus theLC switch can be said to be on for both states. Moreover, the two statesdo not correspond to a maximum and minimum retardance that can beimparted, as was the case in FIGS. 8 and 9. In this case, the secondstate applies a voltage greater than in the first state, and theretardance imparted by the second state is a half wave while theretardance imparted by the first state is a full wave. In some cases, LCswitching is improved by using higher voltages, and thus configuring theswitchable retardance system such that the first and second states bothinvolve finite voltages, or greater than 0V, may enhance switching(e.g., make for faster LC switching). There may be other reasons fordesiring to use non-zero bias voltages, so FIG. 10 demonstrates that theswitchable retardance system can be configured to achieve this goalwhile still enabling a half-wave switching of polarization in a secondstate (V₂), and no change to the polarization in a first state (V₁).

FIG. 11 is a variation of FIG. 10 showing the results of a thicker LCswitch. In particular, the retardance induced by the first state is 4λ,which effectively imparts no polarization change to the light beam, andthe retardance induced by the second state is 3.5λ, which effectivelyflips the polarization of the light beam (e.g., changing right-handcircular to left-hand circular).

FIG. 12 shows a retardance versus voltage plot for four different statesof the LC switch. The first and second states are identical to thoseshown and described relative to FIG. 8. However, third and fourth statesare also shown, where applied voltages are between V₁ and V₂, and resultin retardance between that of the first and second states. As can beseen, this enables four levels of attenuation or beam steering. Thefirst state (corresponding to V₁) can enable full transmission throughan aperture, or no beam steering from an incident direction. The secondstate (corresponding to V₂) can enable full blocking or attenuationthrough an aperture, and a maximum of beam steering for these fourstates. The third and fourth states can result in partial attenuationwhere an aperture at the output is used, and a level of beam steeringgreater than in the first state, but less than in the second state.

FIG. 14 illustrates an LCPG system having an AC bias and optionalfeedback for controlling the AC bias. The LCPG system 1402 can compriseany number of LCPGs as described above, including three and four-gratingsystems. An LC switch 1408 can be used to alter the polarization of anincident light beam at some point within the LCPG system 1402, forinstance, between a first and second LCPG (e.g., in a four-gratingsystem). A first state of the LC switch 1408 can cause the outgoinglight beam to follow a path coincident with that of the incident lightbeam and thereby pass through an aperture 1410. A second state of the LCswitch 1408 can cause the outgoing light beam to follow a path obliqueto that of the incident light beam and thereby impact and be attenuatedby the aperture 1410. The LC switch 1408 can be controlled viaapplication of an AC bias having a first state and a second state, eachcorresponding to a respective one of the first and second states of theLC switch 1408. An AC bias device 1404 can control the AC bias appliedto the LC switch 1408 and an optional bias controller circuit 1406 cancontrol the AC bias device 1404. Optionally, the bias controller circuit1406 may be coupled to one or more sensors that measure an intensity ofthe outgoing light beam at one or more locations and receive feedbackfrom these one or more sensors. These one or more sensors could use anominal amount of light to make readings such that optical throughput isnegligibly affected. The bias controller circuit 1406 could theninstruct the AC bias 1404 to adjust the LC switch 1408 to achieve adesired light intensity at one or more of the sensors. For instance,where the bias controller circuit 1406 receives feedback indicating anamount of light that is on-axis, the AC bias device 1404 could beinstructed to alter the LC switch 1408 until the on-axis light intensityincreases beyond a threshold or is optimized (i.e., a desired level ofon-axis intensity is found). Although FIG. 14 shows two possiblelocations for feedback, certain embodiments may only provide feedbackfrom a single position (e.g., an on-axis position). In some embodiments,measurements of light intensity can be taken for both off-axis beams(e.g., at 1412 and 1414) since, depending on a polarization of theincident beams, off-axis intensity may not be the same at both off-axismeasurement positions 1412, 1414. Optionally, the LC switch 1408 canhave more than two states, each selectable via a different AC bias fromthe AC bias device 1404.

FIG. 15 illustrates a method of attenuating a light beam using an LCPGsystem. The method 1500 can include receiving an incident light beam ata first liquid crystal polarization grating (LCPG), the incident lightbeam optionally being circularly polarized. The first LCPG can deflect apath of the light beam based on a polarization of the incident beam(1502), thereby forming a first deflected beam. Given a certainpolarization, a left circular component of the incident beam may bedeflected off-axis at a first angle while a right circular component ofthe incident beam may be deflected off-axis at a second angle equal tothe first angle, but angled in an opposite direction relative to an axisof the incident beam. If the incident light is not circularly polarized,or at least not perfectly circular, then the incident beam will bedeflected into two first deflected beams. The remainder of thedescription of FIG. 15 assumes circularly polarized incident light, andthus only a single first deflected beam. However, those of skill in theart will recognize that the method 1500 is also applicable tonon-circular incident light and thus those situations where twodeflected beams spread out from the first LCPG.

The first deflected beam can pass through a first substrate withoutdeflection, the first substrate configured to support the first LCPG.The first deflected beam can be received at a first liquid crystalswitch (LC switch). The liquid crystal switch can impart a phaseretardance to the first deflected beam (Block 1504) based on a biasapplied to the LC switch (Block 1506). The first deflected beam can thenpass through a second substrate without deflection, wherein the secondsubstrate can support a second LCPG. The second LCPG can receive thefirst deflected beam, and the second LCPG can deflect the firstdeflected beam based on a polarization of the first deflected beam(Block 1508) thereby forming a second deflected beam. A third LCPG canreceive the second deflected beam and deflect the second deflected beambased on a polarization of the second deflected beam (Block 1510),thereby forming a third deflected beam. The third deflected beam canpass through a third substrate without deflection, wherein the thirdsubstrate can be configured to support the third LCPG. The thirddeflected beam can also pass through a space between the third substrateand a fourth substrate, the fourth substrate configured to support afourth LCPG. The space can be configured to cause a distance between thefirst and second LCPGs to equal a space between the third and fourthLCPGs (Block 1512). The third deflected beam can then pass through thefourth substrate without deflection, and then be received by the fourthLCPG. The fourth LCPG can deflect the third deflected beam based on apolarization of the third deflected beam (Block 1514) thereby forming afourth deflected beam. If the LC switch is in a first state, then thefourth deflected beam may pass primarily through an aperture (Decision1516 and Block 1518). If the LC switch is in a second state, then thefourth deflected beam may be primarily attenuated by the aperture(Decision 1516 and Block 1520).

The foregoing is illustrative of the present disclosure and is not to beconstrued as limiting thereof. Although a few exemplary embodiments ofthis disclosure have been described, those skilled in the art willreadily appreciate that many modifications are possible in the exemplaryembodiments without materially departing from the novel teachings andadvantages of this disclosure. For example, the distance, d, in FIG. 7could be increased to allow a fold mirror or other optical components tobe inserted between the two sub-assemblies. Furthermore, it should beunderstood that analog attenuation may be achieved by setting the LCcell to an intermediate state. This setting directs some of the lightalong the path shown in FIG. 2 and some of the light along the pathshown in FIG. 3, with amounts depending on the retardance of the LCcell.

Accordingly, many different embodiments stem from the above descriptionand the drawings. It will be understood that it would be undulyrepetitious and obfuscating to literally describe and illustrate everycombination and subcombination of these embodiments. As such, thepresent specification, including the drawings, shall be construed toconstitute a complete written description of all combinations andsubcombinations of the embodiments described herein, and of the mannerand process of making and using them, and shall support claims to anysuch combination or sub combination.

In the specification, there have been disclosed embodiments of thedisclosure and, although specific terms are employed, they are used in ageneric and descriptive sense only and not for purposes of limitation.Although a few exemplary embodiments of this disclosure have beendescribed, those skilled in the art will readily appreciate that manymodifications are possible in the exemplary embodiments withoutmaterially departing from the novel teachings and advantages of thisdisclosure. For instance, the herein disclosed systems, methods, andapparatus could be employed for beam-steering applications. Forinstance, using an LC switch having a variety of states and polarizedinput light, enables an output light beam with a selectable exit angle.The use of two or more LC switches could increase the number of exitangles that can be selected, or simplify the circuitry needed to enablesuch beam steering. Further, in a beam steering application, thelocation of the one or more LC switches can be varied (for instanceresiding between LCPG 132 and LCPG 142 or LCPG 102 and LCPG 112).Accordingly, all such modifications are intended to be included withinthe scope of this disclosure as defined in the claims. Therefore, it isto be understood that the foregoing is illustrative of the presentdisclosure and is not to be construed as limited to the specificembodiments disclosed, and that modifications to the disclosedembodiments, as well as other embodiments, are intended to be includedwithin the scope of the appended claims. The disclosure is defined bythe following claims, with equivalents of the claims to be includedtherein.

1. A liquid crystal beam steering device, comprising: a firstpolarization grating configured to direct incident light into first andsecond beams having different directions of propagation than that of theincident light, the first and second beams having substantiallyorthogonal circular polarizations with respect to each other; a liquidcrystal layer configured to receive the first and second beams from thefirst polarization grating, the liquid crystal layer being switchablebetween first and second states for introducing a first and secondretardance, respectively, to the first and second beams; a secondpolarization grating spaced apart from the first polarization grating bya distance D and configured to receive the first and second beams fromthe liquid crystal layer and to alter the respective directions ofpropagation of the first and second beams according to the first orsecond retardance introduced to the first and second beams; a thirdpolarization grating configured to receive the first and second beamsfrom the second polarization grating and to further alter the respectivedirections of propagation thereof; an intermediate region configured totransmit the first and second beams from the third polarization gratingtherethrough; a fourth polarization grating configured to receive thefirst and second beams from the intermediate region and to additionallyalter the respective directions of propagation thereof to provide outputlight; and an aperture configured to transmit a first portion of boththe first and second beams from the fourth polarization grating when theliquid crystal layer is in the first state, and to transmit a secondportion of both the first and second beams from the fourth polarizationgrating therethrough when the liquid crystal layer is in the secondstate, the first portion being greater than the second portion, whereinat least one of the first and second polarization gratings are arrangedon a substrate, the substrate arranged between the first and secondpolarization gratings, and wherein the distance D between the first andsecond polarization gratings is substantially equal to a distance D′between the third and fourth polarization gratings.
 2. The liquidcrystal beam steering device of claim 1, wherein the output light fromthe fourth polarization grating propagates in a direction substantially:parallel to that of the incident light when the liquid crystal layer isin the first state; and oblique to that of the incident light when theliquid crystal layer is in the second state.
 3. The liquid crystal beamsteering device of claim 1, wherein the first, second, third, and fourthpolarization gratings exhibit substantially similar diffractiveproperties.
 4. The liquid crystal beam steering device of claim 1,wherein in the first state the liquid crystal layer introduces aretardance of nλ, and wherein in the second state the liquid crystallayer introduces a retardance of ${m + \frac{\lambda}{2}},$ where n andm are selected from the set including integers and
 0. 5. The liquidcrystal beam steering device of claim 4, further comprising an AC biasdevice configured to selectively apply an AC bias to the liquid crystallayer in order to switch between the first and second state.
 6. Theliquid crystal beam steering device of claim 4, wherein a thickness ofthe liquid crystal layer is such that in the first state the liquidcrystal layer introduces a retardance of nλ, and wherein in the secondstate the liquid crystal layer introduces a retardance of${m + \frac{\lambda}{2}},$ where n and m are selected from the setincluding integers and
 0. 7. The liquid crystal beam steering device ofclaim 1, further comprising one or more trim retarders arranged to oneor both sides of the liquid crystal layer and between the first andsecond polarization gratings, the trim retarders shaped and arranged soas to, in combination with the liquid crystal layer, impart noretardance to the first and second beams when an AC bias device impartsa finite AC bias to the liquid crystal layer, thereby placing the liquidcrystal layer in the first state.
 8. The liquid crystal beam steeringdevice of claim 1, wherein the third polarization grating is configuredto output off-axis beams when the liquid crystal is in the second state.9. A liquid crystal beam steering device, comprising: a firstpolarization grating configured to direct incident light into first andsecond beams having different directions of propagation than that of theincident light, the first and second beams having substantiallyorthogonal circular polarizations with respect to each other; a liquidcrystal layer configured to receive the first and second beams from thefirst polarization grating, the liquid crystal layer being switchablebetween first and second states for introducing a first and secondretardance, respectively, to light traveling therethrough; a secondpolarization grating spaced apart from the first polarization grating bya distance D1 and configured to receive the first and second beams fromthe liquid crystal layer to alter the respective directions ofpropagation of the first and second beams in response to each of thefirst and second states of the liquid crystal layer; a thirdpolarization grating configured to receive the first and second beamsfrom the second polarization grating to further alter the respectivedirections of propagation thereof; an intermediate region having athickness D2 and configured to transmit the first and second beams fromthe third polarization grating therethrough; a fourth polarizationgrating spaced apart from the third polarization grating by a distanceD2 and configured to receive the first and second beams from the thirdpolarization grating to additionally alter the respective directions ofpropagation thereof to provide output light that propagates in adirection substantially parallel to that of the first and second beamsoutput from the second polarization grating; and an aperture configuredto block both first and second beams when the liquid crystal layer is inthe first state, and to transmit both first and second beamstherethrough when the liquid crystal layer is in the second state,wherein at least one of the first and second polarization gratings arearranged on a substrate, the substrate arranged between the first andsecond polarization gratings, and wherein the distance D1 between thefirst and second polarization gratings is substantially equal todistance D2 between the third and fourth polarization gratings.
 10. Theliquid crystal beam steering device of claim 9, wherein the intermediateregion comprises a refractive component.
 11. The liquid crystal beamsteering device of claim 9, wherein D1 is equal to D2.
 12. The liquidcrystal beam steering device of claim 9, wherein D1 is not equal to D2.13. A liquid crystal beam steering device, comprising: a firstpolarization grating configured to direct incident light into first andsecond beams having different directions of propagation than that of theincident light, the first and second beams having substantiallyorthogonal circular polarizations with respect to each other; a liquidcrystal layer configured to receive the first and second beams from thefirst polarization grating, the liquid crystal layer being switchablebetween first and second states for introducing a first and secondretardance, respectively, to light traveling therethrough; a secondpolarization grating spaced apart from the first polarization gratingand configured to receive the first and second beams from the liquidcrystal layer to alter the respective directions of propagation of thefirst and second beams in response to each of the first and secondstates of the liquid crystal layer; a third polarization gratingconfigured to receive the first and second beams from the secondpolarization grating to further alter the respective directions ofpropagation thereof; an intermediate region configured to transmit thefirst and second beams from the third polarization grating therethroughwhile modifying the respective directions of propagation thereof; afourth polarization grating configured to receive the first and secondbeams from the intermediate region to additionally alter the respectivedirections of propagation thereof to provide output light thatpropagates in a direction substantially parallel to that of the firstand second beams output from the second polarization grating; and anaperture configured to block both first and second beams when the liquidcrystal layer is in the first state, and to transmit both first andsecond beams therethrough when the liquid crystal layer is in the secondstate, wherein at least one of the first and second polarizationgratings are arranged on a substrate, the substrate arranged between thefirst and second polarization gratings, and wherein a first distancebetween the first and second polarization gratings is substantiallyequal to a second distance between the third and fourth polarizationgratings.
 14. The liquid crystal beam steering device of claim 13,wherein the first and second polarization gratings are spaced apart by adistance D, and wherein the intermediate region has a thicknessconfigured to separate the third and fourth polarization gratings by thedistance D.
 15. The liquid crystal beam steering device of claim 13,wherein the incident light is characterized by a wavelength λ, theliquid crystal layer exhibits a first refractive index n1(λ) at thewavelength λ, the intermediate region exhibits a second refractive indexn2(λ) at the wavelength λ, and D1 and D2 are related by the equationD1*λ*n1(λ)=D2*λ*n2(λ).
 16. The liquid crystal beam steering device ofclaim 15, and wherein the first, second, third, and fourth polarizationgratings exhibit substantially similar diffractive properties.
 17. Theliquid crystal beam steering device of claim 15, wherein the first andsecond polarization gratings exhibit similar, first diffractiveproperties, and wherein the third and fourth polarization gratingsexhibit similar, second diffractive properties.
 18. The liquid crystalbeam steering device of claim 13, wherein the incident light is incidenton the first polarization grating at a first angle with respect to anoptical axis, wherein the first and second beams exit the fourthpolarization grating at a second angle with respect to the optical axiswhen the liquid crystal layer is in the second state, wherein the secondangle is not equal to the first angle.